Coating metal articles



June 8, 1943. 2,320,998

COATING METAL ARTICLES Filed May 5, 1938 RU 5 BER VULGPKN 1 ZED TOCOATING BLHCK NICKEL COHTING DEPOSITED -FROM BATH CONTAINING K q sLuMmuMMu/rrqy.C-lBe/e a,

Patented June 8, 1943 UNYE' o sraras rA'raur QFFlCE COATING METALARTICLES Murray 0. Beebe, Mount Carmel, Conn. assignmto ScovillManufacturing Company, Waterbury, Court, a corporation of ConnecticutApplication May 5, 1938, Serial No. 206,273

4 Claims.

This invention or discovery relates to coating metal articles; and itcomprises a method of coating aluminum or other metal articles wherein asuitably cleaned metal surface is provided with a nickel coating; all asmore fully hereinafter set forth and as claimed.

Itis frequently desirable to coat metallic articles wholly or in partwith rubber; and in the case of some metals this may be accomplished byvulcanizing the rubber directly onto the metal under suitable conditionsof temperature and pressure. Most metals, however, give only africtioned" coating of no real adherence.

It is generally difficult to form suitable rubber coatings on aluminumbecause an aluminum surface does not bond well with rubber. I havefound, however, that if the aluminum is provided with a suitable blacknickel coating, the rubber may be vulcanized thereon and a durable andsatisfactory bond is then produced; the black nickel coating serving asa linking layer. I have further found that this bond is particularlyeffective if it is formed in the presence of small amounts of copper,which may be either in the alum nous metal itself or in the coating bathfrom which the black nickel is deposited. The latter condition isadvantageous; not only with aluminum and .its alloys but with othermetals also.

In the accompanying drawing I have shown in exaggerated scale by way ofillustration a section of coated article wherein Figure l is a plan viewcut away to expose the successive black nickel coating on aluminum andrubber bonded thereto, and

Figure 2 is a vertical sectional view similarly showing the successivelaminations.

Coatings of black nickel which are satisfactory for my purposes may beapplied to surfaces'of aluminous and other metals by electrolytic actionor by dipping procedures. A suitable bath for the electrolyticdeposition of black nickel is one having the following composition:

, oz. per gal. Nickel sulfate 6 Zinc sulfate 1 Sodium thiocyanate 1 /2The concentrations of the constituents of this bath may be variedconsiderablmfind ammonia or ammonium sulfate may be added if desired.With such a bath. the aluminum article to be coated is connected as thecathode, and nickel anodes are used. Current is applied at a low voltageand low current density; of the order of one or two amperes per squarefoot, for example. An electrolytic bath of the above compositionproduces satisfactory coatings on many'of the commercialforms ofaluminum and its alloys, as well as other metals.

It is often advantageous, and generally less costly, to use anon-electrolytic or dip method of producing the desired black nickelcoatings. One bath suitable for dip-coating aluminum or aluminum alloyarticles, for example, has the following approximate composition:

Water cubic centimeters 1000 Ammonium hydroxide do 50 Sodium citrategrams 50 Nickel acetate do 25 Sodium thiosulfate do 375 Potassiumthiocyanate do- 25 A bath of this composition forms a satisfactory blacknickel coating with an immersion period of about one minute at atemperature of about 130 F.

The bath compositions described hereinabove are quite satisfactory foruse with aluminum alloys and other metals containing amounts of copperup to 5 per cent or more. However, when treating alloys having lowercopper contents, and especially those containing less than about 0.5 percent of copper, it is beneficial to add a small amount of copper salt tothese baths. With the above dip-coating bath, for example, I find thatabout two to five grams of copper acetate per liter may be added, withthe desired result of forminga coating giving good adhesion to orbonding with rubber applied thereto. A rubber coatin may thus bepermanently applied to substantially copper-free aluminum alloys, whileblack nickel ,dip-coatings formed on such alloys in baths free fromcopper salts result in an inferior bond with the rubber.

The addition of copper salt to coating baths is also advantageous in thetreatment of coppercontaining alloys but to a lesser extent.

Another dip bath embodying my invention has the following approximatecomposition:

Water cubic centimeters 1000 Ammonium hydroxide do 62.5 Sodium citrategrams 25 Nickel acetate do 25 Sodium thiosulfate do 25 Potassiumthiocyanate do 25 Copper acetate do 2.5

This bath deposits a satisfactory black nickel coating on aluminum withonly a few seconds immersion, and is effective at moderate temperatures,as low as F.

When the black nickel bonding coat has been produced on the aluminum orother metal surface, either electrolytically or by dipping, as describedhereinabove, therubber coating maybe applied and cured in place by theusual method of vulcanizing with heat and pressure.- The black nickelcoatings have other utilities in addition to their characteristic offorming sound bonds between rubber and the underlying metal. however,and'I consider my coating compositions beneficial and useful, whether ornot rubber is applied over the black nickel coatings obtained therewith.

It should be understood that the metal surface must be cleaned andhomogenized before the black nickel coating is applied, in order toobtain the best results. Various methods of preparing aluminum surfacesmay be employed. but I find that particularly desirable results areobtained if the aluminum is first-etched with caustic soda and thenimmersed in nitric acid. The caustic etching bath may vary considerablyin concentration-over a range from 2 per cent to 25 per cent, forexample-and the temperature of the bath may also be varied over a rangeextending up to the boiling point. I find that very satisfactory resultsare obtained by using a 5 per cent solution of caustic soda at atemperature of 150 to 180 F.. with an immersion time vof about one ortwo minutes, depending on the condition of the aluminum surface. It isdesirable to then rinse oil the caustic as quickly as can convenientlybe done, and to follow this by a dip in nitric acid. The concentrationof the nitric acid may also be varied, and concentrated acid may beused, or solutions as dilute as per cent. After the nitric acid dip,which need last only a few seconds, the aluminum surface should bewashed or rinsed in clean water before it is immersed in the blacknickel coating bath.

After the black nickel coating has been applied to the metal surface aspreviously described, it is customary to clean the surface again if therubber coating is to be applied thereto, This may conveniently be donein a cyanide bath, which eliminates complex oxides from the metalsurface. The rubber composition is then applied with or without a rubbercement, and vulcanized or cured in place at a suitable temperature. 316F. is typical, although the ,optimum temperature for each particularrubber composition varies somewhat.

Various rubber compositions may be applied over the black nickel coatingand, while some work better than others, there seems to be no particularcompounding ingredient necessary to obtain a good bond. Typical rubberformulas A cure for 11 minutes at for this purpose contain to per centof rubber in the form of smoked sheets or rolled crepe: 45 to per centof fillers and pigments such as zinc oxide, litharge. magnesia and clay;I to 3 per cent of sulfur; and 0.5 to 3 per cent of accelerators,modifiers. etc. When a cement is used. it is conveniently applied in theform of a solution in gasoline prior to the application of the rubbercoating itself.

It is to be understood that the specific comcoating.

positions disclosed hereinabove are mentioned bi way of example only,and not by way of limitation, and it is also to be understood that thesecompositions can be variously modified without departing from the spiritof my invention. For example, in the black nickel dip-coating baths.citric acid and ammonia may be substituted for the sodium citratespecified; and. in general. potassium, sodium and ammonium salts may beused interchangeably.

It appears that satisfactory dip-coating baths are always obtained byadding for each liter of water approximately 40 to '15 cc. ofconcentrated ammonium hydroxide: approximately 20 to 30 grams of nickelacetate; approximately 20 to grams of alkali citrate; approximately 20to 30 grams of alkali thiocyanate. and approximately 20 to 50 grams ofalkali thiosulfate, with the addition of about 2 to 5 grams of solublecopper salt, such as copper acetate, when desired. Somewhat wider rangesof concentration are also useful.

Other soluble salts which are compatible with the other constituents ofthe bath may be substituted for the copper acetate and the nickelacetate specified in theabove formulas. So far as I am at presentadvised, however, the acetates are most suitable for this application.The rubber formulas suggested hereinabove may also be modified in wayswhich are well known in the art.

What I claim is:

1. The method of coating an aluminous metal surface which comprisesdepositing a black nickel coating thereon by subjecting said surface tothe action of a black nickel coating bath containing a dissolved nickelsalt, a dissolved sulfur compound and a minor amount of a dissolvedcopper salt. and thereafter vulcanizing rubber onto the said blacknickel coating.-

2. The method of coating an aluminum alloy surface containing less than5 per cent copper comprising applying thereto a black nickel coating bysubjecting said surface to the action of an aqueous black nickel coatingbath containing a dissolved nickel salt, a dissolved sulfur compound andfrom 2 to 5 grams of dissolved copper salt per liter. and thereaftervulcanizing rubber onto the said black nickel coating.

3. A method of coating an aluminum alloy surface which comprisesdepositing a black nickel coating thereon by subjecting said surface tothe action of a black-nickel coating bath containing in solution anickel salt, a sulfur compound and a minor amount of a copper salt, andvulcanizing rubber onto the said black nickel 4. An aluminum-surfacedarticle bearing thereon a coherent black nickel coating characterized byconsisting essentially of nickel. sulfur and a minor amount of copper,and a layer of rubber vulcanized to said black nickel coating.

MURRAY C. BEEBE.

